Don't erase backup metadata when an app is uninstalled

We still retain the data in the backup, in order to support the flow
in which a user has the app and its data is stored; then the app
is uninstalled; then later the app is reinstalled.  We depend on
having correct metadata for the data in the datastore in order to
evaluate its validity for restore-at-install, so we mustn't
forget that metadata just because the app is not currently
installed.

We also now permit the sentinel pseudopackage name "@pm@" as an
argument to dataChanged(), indicating specifically that the metadata
should be scheduled for backup without having to be piggybacked on
another app's requested backup pass.  That lets us now make sure to
schedule a backup pass for metadata-update in response to app
install activity.

Finally, fix a "min instead of max" bug in full backup scheduling
that was causing the OS to ignore the transport's inter-package
quiet time requirement when multiple packages were overdue for
backup.

Bug 21471973

Change-Id: I1dbc260edb91b8deadd2744e273dfa9578b9ef2a